Since the invention of laser irradiation, there has been a need for the protection of photosensitive objects from the intense light generated by lasers. Laser exposure may either be accidental, such as that from a portable range-finding device, or it may be deliberate, such as that from a hostile action. The protection is required for human eyes as well as for sensitive optical equipment, such as specialized cameras. In the case of eye protection, the maximum allowable energy or power incident on the eye has been quantified by the American National Standard for Safe Use of Lasers (Laser Institute of America, ANSI Z136.1-2007), and it depends on the laser wavelength and pulse duration. For example, for lasers in the visible (400-700 nm) and pulses of 1 ns to 18 μs, the maximum exposure is 0.5 μJ/cm2. Herein, references to “eye protection” may also refer to “equipment protection,” unless otherwise specified.
There are several approaches to designing eye protection devices. One possibility is a non-varying device that always blocks radiation, such as dark sunglasses. However, in order for such sunglasses to provide adequate blocking of intense radiation, they would have to be too opaque to be useful to the wearer. Another possibility for non-varying eye protection is to block only a narrow wavelength band of radiation, which would correspond to the incident laser wavelength. Narrowband blocking is accomplished by applying an optical interference filter, comprised of multiple thin films whose thicknesses and refractive indices are selected appropriately. This approach would succeed only if the incident laser wavelength is known, which is not always the case, and only over a narrow range of incident angles.
A better approach than a non-varying optical filter is a device that decreases its transmission in response to changes in the intensity of the incoming radiation. Such devices are termed “optical limiters,” because at low incident light intensities they are transparent, while as the intensity increases, they become progressively more opaque, so that the transmitted light intensity is “limited” to a certain level. Ideally, optical limiters should satisfy several requirements: first and foremost is sensitivity, i.e., their transition from transparency to opaqueness should occur at eye-safe incident intensities. Next, their dynamic range should be high enough that the transmitted intensity is “clamped” at eye-safe levels even when the incident intensity varies by orders of magnitude. Third, for many applications optical limiters should have a fast response time, on the scale of nanoseconds or less. Fourth, they should recover their transparency quickly, if not quite on the nanosecond scale, after the threat is over. Other ideal requirements are insensitivity to the polarization of incident light, a wide acceptance angle, simplicity, environment stability, and so on. Several optical limiting material systems have been investigated in the past. In particular, the requirements of fast response and recovery times have led researchers to explore multiphoton electronic transitions in molecules, such as efficient two-photon absorption in the visible, especially if the excited state absorption coefficient is unusually high. A number of organic compounds with such properties have indeed been identified, e.g., porphyrins and phthalocyanines. However, these molecules are often environmentally unstable, or they can be damaged by very high laser intensities. Even more importantly, according to published papers the incident intensities required to induce changes in transmission are still several orders of magnitude too high (10-100 mJ/cm2), even in the most sensitive optical limiters.
In addition to organic molecules as optical limiters, there have been a few reports indicating that inorganic nanoparticles can also exhibit optical limiting behavior, especially nanoparticles of gold or silver. Unlike in the organic molecules, in the case of inorganic nanoparticles the underlying mechanism for the optical limiting behavior is not well understood, but at least in one publication it has been ascribed to two-photon absorption by the surface plasmons. Prior investigations have studied spherical nanoparticles, randomly distributed in liquid suspension. Until now nanoparticles have not been considered as serious alternatives to organic molecule limiters, because their reported photosensitivity has been no better, and often worse than that of the organic optical limiters: 100-1000 mJ/cm2 have been required to induce a significant increase in absorption.